text:
What types of air traffic controllers are there?
I've heard of many different types of controllers, but I'm not sure exactly what their responsibilities and interactions are.
Here are some examples:

En route controllers
Approach controllers
Ground controllers

What types of controllers exist and what aspects of a flight is each one responsible for?

May I ask why do you ask the question?

See also: What is a TRACON?

In Canada (and I believe similarly in the United States), there are four ratings which can be applied to an Air Traffic Controller license:

Airport Control
Terminal Control
Area Control
Oceanic Control

Within each of those ratings, the responsibilities are sub-divided a bit based on operational needs. For example, controllers at a tower (with an 'Airport Control' rating) would tend to be qualified in all aspects of tower operations including:

Clearance Delivery (issuing IFR clearances or assigning transponder codes)
Ground Control (issuing clearances for aircraft to move to/from the apron and runway areas via a taxiway system
Air Control (issueing clearances to aircraft in the runway environment and in the vicinity of the airport)

Terminal Controllers (if a terminal control area exists around the airport in question) would be responsible for flights in the immediate vicinity of the airport to approximately 20-50 nautical miles. Their responsibilities may include multiple small airports, just a single large airport, or even just one specific operation at an airport (such as arrival control or departure control). These controllers are usually allowed to apply a smaller amount of lateral separation between aircraft due to the dense nature of the terminal environment and the specific nature of their role.
Once the aircraft leave the terminal (if one exists), the Area Controller-sometimes called enroute control-takes over. They are responsible for anywhere between several hundred square miles of airspace to over several thousand square miles in the case of sectors in northern Canada.
In the case of an aircraft transiting the ocean, a specific type of area controller called an Oceanic Controller takes over. They possess slightly different training in order to safely separate aircraft over the ocean in non-radar airspace.

You can find a nice summary here on the FAA website or here on Wikipedia
But generally speaking,
At an Airport (and this very much varies by airport size!)
Note: if there is no tower at an airport (the case for many small airports) aircraft must self announce on the UNICOM Frequency. It should also be mentioned that some airports have towers that are only in operation some hours of the day, they are considered (if open) un-towered at other times.
There may be a Tower controller (if there is a tower): who deals with airplanes on, to and from the active runways.
There could be a designated ground controller who deals with traffic form the ramp to the runway (at KPNE where I fly the tower handles the ground operations during low load times). They will also handle the movements of other things like plows and service vehicles that may be active on or near the runway.
There may be Flight Data/Clearance Delivery who will give the planes their routes before departure (or any taxiing)
Terminal Controllers handle traffic in the radar terminal area (30-50 miles) from an airport.
Enroute Controllers provide information to airplanes in the air such as IFR instructions or VFR Flight Following (Load Permitting)
You can find the frequencies for the various ATC bodies in the Airport Facilities Directory. You can even listen in live to see whats going on at lots of major airports.

In Europe, an ATCO license contains the following:

One or more ratings
One or more rating endorsements
One or more unit endorsements
One or more language proficiency endorsements
A medical certificate

An ATCO certificate shall contain one or more of the following ratings:

ADV - Aerodrome visual (tower control without use of radar)
ADI - Aerodrome instrument (tower control with use of radar)
APP - Approach procedural (approach control without use of radar)
APS - Approach surveillance (approach control with use of radar)
ACP - Area control procedural (area control without use of radar)
ACS - Area control surveillance (area control with use of radar)

For each rating, the following rating endorsements are available:
ADV: None
ADI (at least one rating endorsement required):

AIR (Air Control)
GMC (Ground Movement Control) (without ground radar)
TWR (Tower Control)
GMS (Ground Movement Surveillance) (using ground radar)
RAD (Aerodrome Radar Control)

APP: None
APS (may have one or more rating endorsements, but not required):

PAR (Precision Approach Radar)
SRA (Surveillance Radar Approach)
TCL (Terminal Control)

ACP (no rating endorsements required):

OCN (Oceanic Control)

ACS (can have one, but not two, rating endorsements):

OCN (Oceanic Control)
TCL (Terminal Control)

In addition, an ATCO will need a unit endorsement, which is the authorisation to provide air traffic control service for a specific sector, group of sectors and/or working positions under the responsibility of an air traffic service unit. Basically, a certification that you know the local regulations at the unit you are working at.
A language proficiency endorsement is also required in English and, if applicable, in the local language if required.
Furthermore, there are instructor ratings available, but I think that it beyond the scope of your question.
Then of course all controllers need to hold a class 3 medical certificate for their ATCO license to be valid.
As you can see, with the number of ratings and rating endorsements available, there are a lot of different possible combinations. So the answer to "what types of air traffic controllers are there?" is not a simple one. Regardless, I hope the above gives you a general overview of how ATC licensing is structured in Europe.

The previous answers detail the air traffic controller types and their roles very well. However, I find this diagram quite helpful to visually recall the airspace divisions.
ACC: Area Control Center
APP: Approach/Departure Control
TWR: Tower
UTA: Upper Control Area
CTA: Control Area
TMA: Terminal Control Area
CTR: Control Zone

hi what mean stack area in the picture?


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Has there been research into alternatives for the vertical stabilizer?
Are there some activities in research for replacing the vertical stabilizer with a turnable horizontal stabilizer? (flight direction stabilization like the birds)

The probably has been such research. However, birds have proven to be a bad model for manned flight. The problem with that concept you describe is that it is a cross between two approaches: (1) The surface approach to stability and (2) The variable shape approach to stability (used by birds).

related: http://aviation.stackexchange.com/questions/12055/why-isnt-it-possible-to-locate-the-rudder-in-front-of-an-airplanes-center-of-g/12103#12103

There have been various (and of varying degrees of success) some flying wing designs that have made it into production with no vertical stabilizer. However they had no turnable horizontal stabilizer either. The B-2 Spirit comes to mind,

(source)
The closest thing I can think of that has no proper vertical stabilizer but still does have an epenage is a V tail design, a variety of planes have been made with them over the years including the popular early Beech Bonanza models.

(source)
I am not sure of any planes that have ever been produced with such a tail nor any research projects on such a design but that does not preclude their existence.
A general note on aircraft research and design, the various flying wings and V tail planes built over the years have had issues and been met with hostility by some critics. This is not to imply that they are poor designs however the flying wing does have its share of instability inherent to the design. The V tail gained some negative reputation on the early Bo's because many of them crashed. However many people account this to low time pilots in overpowered (for the time) planes.

The Fouga-Magister had a V-tail, and has been used a long time by the national military aerobatic team (Patrouille de France) and other aerobatic groups, as well as for jet training. I believe the behavior of such tail must have been quite satisfactory for these uses.

I thought ALL aircrafts needed a vertical stabilizer... How is the B-2 supposed to fly ?

As far as I know. The design of the B2 is inherent unstable and will crash immediately, without it's computer-system which fixes the unstability through permanent  interventions ("bird brain"). Therefore it uses a four time redundant fly-by-wire system. The control surfaces are always moving a around a little bit, even if the pilot is doing nothing (correct?).Turning off the electronics on a modern passenger jet is also not good, but you can survive. At least you will not drop out of the sky instantly, because their designed is stable and the jet will glide itself to some degree (correct?).

The B2 photo shows the outboard split ailerons whose variable drag replaces the yaw force that a vertical rudder would provide.

@Matt: Carefully.

What you describe implies the use of artificially enhanced stability. There have been projects which did away with a vertical surface completely, lately to improve the low-observable characteristics. However, in each case you replace something that automatically produces a stabilizing effect with something that needs to be positioned actively in order to have a stabilizing effect. So far, the preference for safety has meant that active stability augmentation has not made inroads into regular aircraft - the added failure mode is simply not worth the gain in aerodynamic qualities.

An early example would be the Horten II motor glider (picture source)

A more recent example is the unmanned X-36 (picture source)
In both cases the designers simply left the vertical tail off and used other means for directional stability. Your proposed moveable tail surface has never been tried, as far as I know.

Is that a propeller at the rear center of the Horten?

@TomMcW: Yes, a fixed-pitch wooden propeller. The engine was a Hirth HM-60 with 60 hp. The Horten VII even had two propellers and two Argus As-10 C engine with 240 hp each.

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Is solar power used (as auxiliary power) on commercial aircraft?
Are there any commercial aircraft that use solar energy in terms of auxiliary (not as main power source, as of now) power source ?
If Yes, In what forms is the solar power used?
If No, Why isn't solar energy used in commercial aircraft as an auxiliary power source?

Probably not on a commercial airliner, because solar cells don't have a good energy/weight ratio, they need a battery to store energy, and are fragile at Mach .7. In addition they don't work well within clouds and rain. On Earth, solar power is limited to 250 W/m² at 100% efficiency (so maybe a mean 50 W/m² in operational conditions).

Yes. When the aircraft is sitting at the gate on a bright, sunny day, it uses solar energy to overheat the interior!

The sun's energy is a max of about 1000 W/m2 at sea level (sun directly overhead, clear day). A solar power installation may have around 15% efficiency, yielding a max of 150 W/m2. Let's just see roughly what that gives us for a 737, to see if it's even that much.
The wing area of a 737 is 124 m^2, and the fuselage is 42 m long and 3.76 m wide. This gives a total area of 280 m2 if the sun is directly overhead. Covering all of this with solar panels gives a total (extremely optimistic) output of 42,375 W. The fuselage tapers at both ends though,  and needs to have openings for the windows, antennas, etc. The wings will probably only have the fixed panels covered in solar panels (not flaps, slats, etc) so you would be looking at significantly less for a realistic installation. The Solar Impulse 1 was in the same ballpark (200 m2 for a peak of 45 kW).
The APU on a 737 is rated at 90 kVA. Of course you will usually use less than that, but we are looking at solar only providing maybe 20% of the capacity for any considerable part of the day. Since solar is not always available, you cannot even downsize the APU.
Another important function of the APU is to provide bleed air. This can be used to run the air conditioning packs and to start the engines. Photovoltaic panels would be a very inefficient way to provide bleed air.
An airplane will probably look strange being covered in solar panels, but covering so much area also causes some issues. Inspecting the aircraft for damage will be difficult. This means the solar installation should be easily removable (also for repairing damage to the solar cells), and the structure may need to be more damage tolerant. The panels would also need to be attached well so that they would not depart the plane even if damaged. This all adds more weight.
You would be adding a lot of weight to install solar that only helps a little bit sometimes. You may get a small benefit by reducing the generator load on the engines, but probably not nearly enough to make up for the increase in weight.
Airlines are big on reducing fuel usage, and are willing to add weight to save fuel overall (such as with winglets), but we haven't seen solar put to use on any airliners yet. The most efficient way overall to provide regular power is to extract it from the engines that are already running. For auxiliary power, a small turbine generator, which can provide both bleed air and electricity. And for emergencies, a RAT.

No. The APU has to work on night flights as well as it works during the day.  (Granted, it can be inoperative and deferred for either, but the general expectation is that the equipment, when operative, is available 24x7, rather than 12x7.) Anything that's always inoperative from sunset to sunrise is a novelty at best, and excess weight for sure.

This may be misleading, aircraft fly quite regularly with inop APU's, or they are sometimes powered down in flight.

While an aircraft certainly CAN be dispatched with an APU inop, it would be wasteful to build one knowing in advance that it will be inop essentially half of the time at best.  The purpose of an APU isn't to be run continuously during flying (ETOPS excepted), but to be available as a backup power supply if a generator dies.  Generators die at night just like during the day.

Right, but consider the APU is not on the MEL for most commercial (non-ETOPS) aircraft and is a deferrable maintenance item. The actual back-up for an "all-generator-out" scenario is the RAT, some aircraft are even equipped with squat switches that prevent APU operation in flight (typically smaller aircraft). Some aircraft won't allow an APU start when the engines are dead (DC-9) because the batteries are required equipment (30 minutes without generators). Either way though, having an inop APU is more common than you think.

Who wants a RAT replaced with something that can't be used at night?!? The OP asked why solar power isn't used in commercial aircraft, and the answer is that adding weight + complexity for something that doesn't work at night, isn't viewed is a positive addition to the aircraft. All the discussions of DC-9 systems and Boeing APU's and whatever else doesn't change that central fact.  ((BTW, if an APU is "not on the MEL" for a given aircraft, then it would have to be operative for the aircraft to fly... not on the MEL = no MEL relief = can't defer it.  Think you meant, not REQUIRED by the MEL.))

Yeah, I worded that wrong about the MEL. I wasn't suggesting replacing the RAT or the APU, I guess I was just trying to say that when you said "The APU has to work on night flights..." it sounds like there is some rule about an operational APU, which could be misleading. I'd say it would be better if you just said "The APU works on night flights as well..." to clarify. I think we are agreeing but I'm just not wording things right, its early :)

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text:
Why aren't ceramics used in turbines?
I have seen that ceramics take a lot of heat to melt (3240º F). So, I'm wondering, why aren't ceramics used as the compressor levels in turbines? Is there something to do with the strain? or is ceramic too heavy? Is it something to do with the way the crystals align?

Related: What material is used to make the hot sections of jet engines?. Ceramic would be used in the hot section, not in the compressor section.

And also Why are aircraft turbine blades made of nickel alloys?, and ceramic matrix composites.

@fooot I see. Thanks for the help. However, suppose ceramics were used not only for the hot section. For example, how about the exhaust nozzles

Suppose there were an ignition source in the compression levels. Should a ceramic be used then?

The exhaust area is not really heat limited, nickel or titanium alloys do just fine. Ceramic is brittle so not well suited in most applications. CMC is still fairly new and may make more cost sense in the future. There would typically not be any ignition in the compressor sections, but anywhere handling ignition gases, like the turbine, would be limited by heat and thus benefit from ceramics.

@fooot Sorry. I should explain a little. The reason I'm asking is because I am creating an aircraft, and I'm looking for materials that will survive under the heat of a full jet fuel ignition. Since most turbines don't ignite, I need to look for more durable materials for my engine, and I'm trying to find whether ceramics would work for my purposes. I guess not.

Plain ceramics, not really. But the ceramic composites I linked to are starting to be used in turbines and they have weight and temperature benefits.

@mins I'm planning on using the compressors AS the combustion chamber. Right after(or in) the last high pressure section, I would set sparkplugs or another ignition source to light the fuel on fire, which should give a thrust boost.

@Nicholas there is a good reason all the experienced turbine manufacturers (and anyone who understands them) don't do combustion in the compressor. I suggest you go think why that is.

On the other hand, @Notts90, sometimes great breakthroughs come from those who go against the grain. (To be honest, I have no idea why combustion isn't done in the compressor, but I can think of a couple of reasons...)

@mins if the combustion was done before the last stage of the compressor what would be the point of any additional stages providing further compression? The additional stages would be redundant, you want turbines after combustion to recover energy. On top of that making components heat resistant adds a lot of weight. You want to have to do that to as little as possible.

@mins apologies, it's early here and brain hasn't fully engaged.

Ceramic materials are starting to be used in jet engines.  E.g. the LEAP-1A is using a few ceramic matrix composite in the high pressure turbine.   (http://aviationweek.com/mro/ceramic-composites-enter-service). Future designs will likely expand the use of CMCs (e.g. http://www.geaviation.com/press/ge90/ge90_20150303.html)

These highly desirable properties of ceramics have as yet been largely disregarded, due to the perceived low toughness and brittle failure demonstrated by traditional ceramic strength.


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What is a plausible scenario for aircraft sabotage for a science fiction novel?
I am writing a science fiction novel and my characters are about to board a private jet - they will be leaving Virginia and flying to CERN in Switzerland. Some factions are not happy with what they are attempting to accomplish and the plane is sabotaged. Here is the important part - one of my characters is an alien with limited telekinesis ability. If something is "stuck" then he will be able to save the plane by moving it, with his mind (not the plane itself - that is beyond his ability).
So my question is: What is a plausible mechanical failure that could be induced in advance by someone on the ground who was trying to prevent them from getting to CERN, that could be discovered by the pilot/crew during flight and that my alien can correct in time to save everyone using his telekinesis ability?
In the book's plot, the alien would have to be able to see it in order to move it...I know nothing about aviation so I don't know how to make this scene at least somewhat plausible -any suggestions would be deeply appreciated!

I can't give a complete answer but I can come up with some helpful criticism:  1)  most pilots do a complete pre-check of the airplane before taking off.  This includes moving all the control surfaces, checking fuel, running up the engine, etc.  2) Most of the really important things on an airplane can't be seen, violating one of your constraints.  3) Redundancy on airplanes makes it almost impossible to take out one thing and cause a failure during cruise (takeoff would be a little easier to sabotage).  It's much easier to force an emergency landing than to crash an airplane.

While the context is certainly more in Worldbuilding's territory, the specific question here does seem to me to be on-topic for Aviation. Just because it would also be on-topic for Worldbuilding, doesn't mean it's necessarily off-topic here. As such, I'm voting to leave open. For the specific part of the question that is asked here, it seems to me that Aviation would be more qualified to answer it than Worldbuilding, since it appears to be about technical plausibility.

Basing the idea on:  https://en.wikipedia.org/wiki/British_Airways_Flight_5390
If someone deliberately installed the wrong size screws in the windshield, they could be missed in pre-flight checks.  You could expect the windshield to fail at altitude, either crashing the plane or ending in an emergency landing at the closest field.   The screws are visible from the cockpit, so your telekenisis could work.  They're pretty small.

I'd recommend you look at actual flight crashes to determine possible scenarios.  The one @ablenky linked is good, or you could try something with uncommanded control surface deflection due to a hydraulic system failure.  Anti-icing system sabotage (ice on wings has contributed to plane crashes) is also a possibility if the saboteurs know that icing conditions are likely on the route.

Another interesting crash is: https://en.wikipedia.org/wiki/Continental_Express_Flight_2574
Screws very high up on the tail were removed, but not replaced during maintenance.   However, those aren't visible from inside the plane.

One more:  https://en.wikipedia.org/wiki/Alaska_Airlines_Flight_261
The jack screw (inside the tail, completely hidden from view) did not have enough grease on it, and got stuck.  (doesn't meet all your criteria, but is interesting)

On my Cessna 177 there is a fuel drain hose that is tucked between the aileron and the wing. If this drain line's retaining bracket were removed or loosened, the tube could get stuck with the aileron. Ailerons are pretty easily seen from the inside and this exact thing has caused aircraft accidents, it's easily missed in preflight, and some older aircraft may not have the retaining bracket.

@Ron Beyer Thanks for providing that scenario! What do you think of the C-21A and does that or something similar work for it? I am using a military type because this trip is at the behest of the U.S. government and NASA is also involved.

@ngolden NASA isn't military, they do use a lot of typically "business" grade jets, so don't limit yourself. Usually a lot of aircraft have "things" running inside the aileron locations that could jam an aileron, but it is a rather rare occurrence.

@Ron Beyer - thanks for pointing that out! Simply put - I need to tranport 6 scientists from Dulles to Switzerland with the scenerio I have outlined. In my limited knowledge I realize I need a long range transport but other than that - I am open to whatever would be appropriate. I thought by using a military transport it is funded by the U.S. government so seemed a natural fit since the scientists are government funded...but if you have a better idea i would love to hear it.

@ngolden Any of the jets that are long range on this list are appropriate. For example the Gulfstream III is NASA's "executive jet" and could make the trip

@Ron Beyer Thanks for the list! It seems to me the Gulfstream III may be a better choice, thank you! The C-21A is actually a military variant of the Learjet 35 but is not on the NASA list so the Gulfstream would be much more plausible. Now i just need to figure out that recoverable failure...

While the discussion in the comments here may be helpful for writing a book they're really more of a group discussion than comments on the question itself - the whole topic may be better suited to chat  (either ours or the room for worldbuilding & ping a few folks from Aviation to join the conversation). There's more nuance here than can be reasonably worked out in simple Q&A and chat is generally better for discussions.

If the alien has to see it to fix it, that rules out anything not visible from the windows (i.e., tail, landing gear, etc.)
If the alien is a passenger, the only moving parts that come to mind are the ailerons.
Have your saboteurs cause a complete failure of the aileron controls (In a Gulfstream 3 I think it's cables with a hydraulic boost, but I could be wrong).  A semi-cut cable won't be caught on a preflight (who takes apart a wing?), but could break over the ocean.
Your alien could save the day by directly manipulating one of the ailerons to keep the plane level.
I don't know the forces involved - I hope your alien is really strong.

@ Dan Pichelman - thanks for the very informative response!

Based on what happend with BA 5390:
If a saboteur installed the wrong size screws on the windscreen, I think the pilot could easily miss that during the pre-flight inspection.  The screws would hold during takeoff and the climb, but would begin to work their way loose at cruising altitude.
The plan would be that the windscreen would come out midway in the flight, and either crash the plane, or force them to descend rapidly and make an emergency landing (which would be very difficult over the middle of the Atlantic!)
Your telekinetic alien can see the screws from the cockpit, and hold them in place for the rest of the flight, averting tragedy.
In the comments, I listed two other accidents that involved relatively minor maintenance errors resulting in crashes, but they don't meet your other criteria, such as the ability to directly see the problem from inside the plane.

thanks for the idea! An interesting scenario...I'm putting it on my short list of possible scenarios to brainstorm.


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What is this odd large sign near Aalborg airport/millitary airport?
I have recently moved to Aalborg in denmark and while riding my bike each morning I cycle past these large signs pointing out into the water. I assume they are for aircraft at the nearby millitary/civillian airport because of how large they are and how they point directly out into the water in a relatively calm stretch of water on the edge of the city so nobody could see them.

The signs are quite large, about 1-1.5m in diameter and the poles are about 6 or 7 meters tall
The one interesting thing that made me think maybe its not to do with the airport is how it's actually quite far away (and also not lined up even slightly with the runway). In fact, they are almost perpendicular to the runway, about 4km away across the water.

I'd love to hear your thoughts on the matter, and apologies in advance if they turn out to be some other kind of markings. I am very curious to know!

My first thought would have had them as some kind of marine traffic/shipping signage rather than aviation related... not sure though!

I don't think it is aviation related. Note that it is apparently made of wood & unlighted. Also, its size is relatively small if it were to be observed from a flying aircraft.

It's a maritime navigation aid. I'm fairly rusty, but I'm pretty sure it's a form of lead mark. When seen from the water, as you travel towards the mark, if you keep the diamond centred between the circled dots, it will have you travelling in a straight line down the centre/safest path of a channel.
It will normally be associated with a channel through shallow water, or a hazardous bend, or promontory, or breakwater, or a safe entrance or exit course for a harbour, or similar. Sometimes there may be a progression of such marks to enable zigzagging along an irregular waterway with a minimum of course changes.

Related at the end of the page, the same signals in red, and with roughly translated names: Directional aid lower/upper.


text:
Why do you have to keep your seat belt fastened after landing?
What's the point of the usual "please keep your seat belt fastened until the aircraft stops completely (at the gate)" in large passenger aircrafts? Since the plane is on the ground, it is not moving very quick. Even if it stopped abruptly, the seat belt won't prevent me from banging my head against the seat in front of me since it's a two-point seat belt.
The only reason I can think of is if two aircraft collided with each other, which could potentially generate enough force for the seat belt to be useful. Though, I could argue that when a plane is taxiing to the gate, it has to abide to a certain speed limit, which would render any kind of collision not serious enough to be prevented by two-point seat belts.

If not, people would stand up and move around.  Chaos would ensue, not the least to say a sudden stop could cause people to fall and the frail and elderly to break limbs and so on.  Do you undo your car seat belt whenever you move at slow speed?

example: http://avherald.com/h?article=432c535e&opt=1024

@Simon That's a good point about keeping people seated. I was trying to think of an accident prevention use case but crowd control seems like a likely motivation for this rule.

@Federico Thanks for the report, though it doesn't illustrate how a seat belt could have helped in this case. If not using a three-point seat belt system, stopping abruptly still causes your upper body to slam into an object in front of you.

taxiing aircraft are moving at speeds similar to cars in city traffic (on high speed taxiways they can go pretty damn fast too by the way), braking hard (you don't need a collision) will cause anyone standing up to lose their footing. "Not moving very quick" is in many cases an optical illusion, you're high up on a huge taxiway, everything around you is big, and thus everything seems to be moving slowly.

"Why do I have to keep my seatbelt on in the car?  Even at a stoplight?" Because you never know when something else might hit you.

@falstro: In fairness, geometry matters. Driving a car at 100 km/h in city traffic is a whole lot more dangerous than driving a car at the same speed on an abandoned runway. If the latter doesn't feel dangerously fast, it's not an illusion.

Just to illustrate @abelenky's point

@MarcksThomas sure, geometry matters. Big moving things have a lot more energy and momentum, and it takes more effort to slow them down. That doesn't change the fact that there's a lot of momentum going on though. See TomMcW's link for an example. The optical illusion part of my comment applied to the "Not moving very quick" quote which was in the original question but edited out within the grace period (i.e. doesn't show up as an edit)

Ground operations are just as dangerous as air operations. Strictly speaking the deadliest aviation incident actually happened on the ground (not saying seatbelt played a role or not however severe accidents do happen on the ground). The comments cover things well, (keeping people seated, keeping people from accessing over head compartments etc.) Planes are not safe simply because they are on the ground, they are only safe from flight related incidents and once on the ground they are susceptible to ground related incidents. By some estimates 27,000 ground incidents happen a year. The fact remains there are plenty of things to hit on the ground and plenty that can go wrong in taxi. These accidents are classified by ICAO under (RAMP) and (GCOL) codes (and potentially others). Boeing has a breakdown of accidents by flight category (taxi and ground ops make up about 10%).
Furthermore here in the US under the FAA FAR 91.107 as well as FAR 121.311 in the commercial case(basically the same wording) requires it

...(3) Except as provided in this paragraph, each person on board a
  U.S.-registered civil aircraft (except a free balloon that
  incorporates a basket or gondola or an airship type certificated
  before November 2, 1987) must occupy an approved seat or berth with a
  safety belt and, if installed, shoulder harness, properly secured
  about him or her during movement on the surface, takeoff, and landing.
  For seaplane and float equipped rotorcraft operations during movement
  on the surface, the person pushing off the seaplane or rotorcraft from
  the dock and the person mooring the seaplane or rotorcraft at the dock
  are excepted from the preceding seating and safety belt requirements....

Also possible that if passengers were allowed to stand up, they would remove their baggage from the racks. This could be a problem to have obstacles all over the floor (or in the hands of the passengers) if the cabin had to be evacuated due to hydraulic fluid leaking on the hot brake discs.

@Dave I would add Mins comment to your answer.  It's another excellent reason to keep people belted in.

Added and citied

Twice, I've had vehicles (the tractors that tow the baggage carts) cut sharply in front of me as I've taxied toward the gate.  The individual driving the tractor was either oblivious or reckless -- either way, foolish in the extreme -- but no matter what his issues are, I'll do everything I can to avert the risk of a collision.  And that means using the wheel brakes that can stop the aircraft when it's moving nearly 200 mph... and using their full braking capability, right now.  If anyone were standing, he'd have a face full of bulkhead or floor before he had an opportunity to react.  If he were retrieving luggage from an overhead bin, the chance that he'd be able to control the bag are probably pretty slight, and now it's a projectile coming down on the back of somebody's head.
Even our Flight Attendants stay seated as we taxi in to the gate, simply because of the risk that the pilot may need to bring the aircraft to an immediate, even violent, stop with no warning.  It's pretty rare, but when it happens, it happens hard.  Taxiing out for takeoff, you've left the congested gate area behind, and this risk is lower, but after landing, you're heading to the most congested area with the greatest opportunity for something or someone to intrude into the path of the aircraft.
So, yes, by all means, please do follow the Flight Attendants' instruction, and example, and remain seated with your seatbelt fastened until we're parked at the gate.  The face or other body parts you save, might be your own!

The wheel brakes that can stop the aircraft from 200 mph don't actually have that impressive braking action. The limiting factor for stopping is not the deceleration rate (the runway needs to be long enough for take-off anyway) but the kinetic energy the brakes convert to heat. From 50 mph, any old bus can stop in a bit shorter distance than most aircraft (and people do stand in buses).

@JanHudec The deceleration rate of brakes is vastly important for an RTO, which is also the limiting factor for runway length. An emergency stop in a bus would throw people around too. Heck, regular stopping can knock people over which is why they ask you to hold on.

A plane that has just landed is actually in a new and vulnerable condition. The passengers may finally be relaxing because the wings didn't fall off and the Italian mathematician in has packed away his terrifying scribblings, but the aircraft has entered a new environment with new risks, and the crew are certainly not thinking "There, done that, work's over."
A stationary or slow-moving aircraft is at risk from a fuel, oil and even other fluid leaks in a way that one that's flying isn't.
A newly-landed aircraft has new, different stresses on its structure.
One that's just landed has hot engines, brakes and tyres. Tyres in particular, but also other components, can and do catch fire.
Unlike when it's in the air, an aircraft heading to the terminal is in close proximity to buildings and other obstacles, some of which also move around themselves.
For any number of reasons aircraft may need to be evacuated, or manoeuvred violently shortly after landing.
Those are not circumstances in which you want 200 people standing up, blocking each other's view of the cabin crew, talking loudly, pulling luggage out of overhead compartments, putting objects in the aisle.
The safe place for people to be is sitting quietly in their seats, until the doors are opened and they are invited to get up and leave.


text:
What happens to cabin pressurisation when the engines fail?
I was reading about the cabin pressurisation mechanism from Wikipedia and another article here. According to both of them, the aircraft's engine plays a vital role in the  pressurisation process.

My question is:
I have come across many air crash investigation articles mentioning that all the engines of an aircraft had failed, but the pilots were able to glide the aircraft and safely land it with no casualties. How is it possible for everyone onboard the aircraft to survive without cabin pressurisation (with all engines out) ?
UPDATE: What if a safe landing area is a bit too far? After losing altitude, the aircraft can't obviously gain altitude. Won't the plane crash due to low altitude (the descent is fast) gliding?So, isn't it better to maintain (sufficient) altitude to reach an emergency destination? I hope the altitude can be maintained provided cabin pressurisation is intact. But, how is this achieved in commercial aircrafts?

TLDR: the APU kicks in and everything stays alive long enough that the pilots can respond to the emergency.

I guess pilots close the exhaust and the pressurization should hold long enough (time to glide to a flight level where pressurization is not needed)

What happened with Air Transat 236?  That craft was out of fuel somewhere near FL345.  Nothing I've read about it suggests that they did anything other than max glide before the approach.

Your "update" is an entirely separate question. Please ask it independently, instead of having multiple questions in one. This is a Q&A, not a forum with threads.

@Nij My update points to the same problem of loss of cabin pressurisation when all the engines are out. It just adds a scenario of when a safe landing area is far and will be impossible to reach when the aircraft drops down to lower altitudes in order to get breathable air (since dropping down to lower altitudes is the only way of saving the passengers).

Asking how commercial pilots will aviate in a situation where cabin pressure is lost, is not the same as asking what happens in the aircraft itself to deal with that cabin pressure loss.

Closing the outflow valve would seal up the plane, so it wouldn't depressurize immediately. Once the pressure falls below a safe threshold the oxygen masks would drop. The masks are good for about 15 minutes.
Air Transat flight 236 which ran out of fuel over the Atlantic holds the record for the longest unpowered glide in an airliner. It lasted about 19 minutes.
The APU can provide pressurization, but if you're out of fuel that won't work either. Air Transat was running off the RAT and I doubt that can provide pressurization. BUT if you factor in the outflow valves being closed and the last part of the glide being below 10,000 feet the oxygen masks will provide ample time for the passengers to stay safe.

If the engines of the aircraft fail, the APU can supply the required air for some time. During this period, the pilots will initiate descent to lower altitudes (10000' or lowest safe altitude permitted by terrain) where the passengers can breathe without cabin pressurization.
The cabin pressurization system is controlled by an outflow valve, which is shut in case the pressure differential goes above certain value or the pressure altitude rises above a certain value. This prevents the loss of air in cabin. The air coming to the cabin is prevented from going in the opposite direction by check valves.
Commercial aircraft usually carry supplemental oxygen, which is fed via masks in the cabin (which drop automatically once the pressure altitude reached 15000' or can be dropped by crew before that). As the supply is limited , the crew (who will don their masks first) will get the aircraft to low altitudes.

This creates an interesting dilemma, though: do you start an emergency descent without engines? Or would you rather not lose altitude you know you never can get back? Obviously, you need to keep the passengers alive, but do you need to keep them conscious?

Some aircraft also have a maximum altitude for starting the APU, below typical cruising altitude.

@JörgWMittag You need to keep then conscious so they can follow emergency procedures and an eventual evacuation

What if a safe landing area is a bit too far? After losing altitude, the aircraft can't obviously gain altitude. Won't the plane crash due to low altitude (the descent is fast) gliding?

Oxygen for breathing is all that is required to survive the decent. That's what all the little masks that drop down are all about... don't forget to pull the little string. It may not be enough to keep you conscious in the event of a full loss of cabin pressure at cruising altitude but it will keep you alive.

If it's not going to keep anyone conscious, what's the point? How will the pilots pilot the aircraft? And also remember that you will lose consciousness in a maximum of 15 seconds (due to hypoxia).

@steam7137 The pilots' oxygen system is separate from that of the passengers and delivers oxygen at a higher rate than that of the passengers. Also, while the passenger masks are flimsy affairs that allow considerable leakage, the cockpit masks are durable and have relatively little leakage. Also, individuals vary greatly in their tolerance of lack of oxygen. Additionally, recovery when breathable altitudes are reached is but a matter of seconds for a reasonably healthy person.


text:
Why does this 757 have a propeller engine attached to the fuselage?
I saw the following photo of a Honeywell Aviation Services Boeing 757-225:

If it's not a fake, the airplane has a propeller mounted on a pylon on one side.
What's the purpose of such strange design? Why does it need a propeller engine, if it already has two jet engines? Or is it a permanently deployed RAT?

The plane doesn't need the propeller engine, the propeller engine needs the plane to bring it into the appropriate test conditions.

The livery should be a bit of a hint here. Honeywell is a major manufacturer of avionics and also engines and components.

Related: https://m.flugrevue.de/sixcms/media.php/11/thumbnails/GE%20Aviation%20-%20GE9X%20Erstflug%2013.%20Maerz%202018%20%28GE%20Aviation%29.jpg.8649690.jpg and https://aviation.stackexchange.com/q/19669/1084

There's a decent article on the plane here: https://www.flightglobal.com/news/articles/honeywell-757-testbed-chases-storms-in-asia-pacific-449766/

@JanHudec never mind the company name, the fact that it says “Flight Test” was the clue.

Now that would be a really big RAT...

That is one cluttered livery. It shows off Honeywell Wi-Fi, wheels and brakes, Wi-Fi, a new avionics suite, Wi-Fi, their APU, and did I mention Wi-Fi?

@Notts90, yes, but the “Flight Test” text can only be seen in the full size image, while the large Honeywell logo on the tail catches the eye immediately even on the embedded scale-down.

One sort of turning, two burning.

From the description below the image you linked:

The Honeywell, Boeing 757 test bed appeared on static display for a few days before departing half way through the event.

A testbed aircraft is an aircraft used for flight research or testing new equipment such as engines.
You can see other photos without engine or with a jet engine instead of a propeller one.
The engine is not needed to fly, but is being tested, and apparently that position has been deemed a good solution to the problem "where do we attach an extra engine we need to do tests on?".

The airplane doesn't "need" that engine.
It is a test platform for testing the engine during flight.
Obviously, you don't want to rely on the engine you're testing, in case something goes wrong!  So the plane flies with 2 large turbofan engines, while the engineers are testing the smaller engine mounted up front.
If you look closely, you can even see the airplane is labeled "Flight Test".

Testing a new engine design can only go so far on the ground. At some point, it has to be tested in conditions representative of actual use - in other words, in flight. You don't want to be dependent on an untested engine to get you airborne and back on the ground safely. The solution to this problem is to mount the test article on an aircraft that is entirely airworthy regardless of the operational state of the engine under test. That way the test engine can be put through its paces without compromising the safety of the test crew.

I guess this  is the reason why the test bed is fairly massive B757, a smaller airplane would be in much bigger trouble in case the tested engine misbehaves, like goes in reverse.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Would a large airliner such as a 747 or an A380 survive an inversion?
Could any large airliner or GA aircraft survive a horizontal roll? Or would a large aircraft break up from structural damage?

not properly an horizontal roll, but pretty close: http://aviation.stackexchange.com/questions/149/2698

Could it? Yes.
Tex Johnson famously performed a roll in a Boeing 707. If performed properly it is a 1G maneuver (the airframe experiences +1G stresses through the maneuver - as far as it's concerned nothing unusual is happening).
Should you attempt such a maneuver without aerobatic training? No.
Should you attempt it in an aircraft not certificated for aerobatic flight? Absolutely Not.
This is one of those "If you do it wrong the wings fall off" maneuvers, and chances are you're not Tex Johnson.

Also should be noted that A380 won't let you do it as it has both pitch and roll angle limit. B777 and 787 won't let you do it either, though I don't think they ported the system to 747-8.

I think the question is not asking about an intentional roll, but a survivable recovery from a inadvertent inverted attitude.

A properly executed roll isn't an inversion as far as the airplane is concerned.  A ground observer will see the airplane roll and be upside down for a time, but the airplane in a 1G roll will think it is right side up the entire time.  An observer in the airplanes reference frame would not experience the roll as the gravity vector is a constant 1G pointed through the bottom of the seat during the entire roll (see this video).
As pointed out in this other answer, don't do this in an airplane not certified for it.  Yes, it can be done, but if not flown perfectly the outcome is likely death in a large turbojet airplane.


text:
For training with a grace month, is it legal to fly in the month after training expires before going to school?
Most 135 training/testing says something like this:

§135.343   Crewmember initial and recurrent training requirements. No
  certificate holder may use a person, nor may any person serve, as a
  crewmember in operations under this part unless that crewmember has
  completed the appropriate initial or recurrent training phase of the
  training program appropriate to the type of operation in which the
  crewmember is to serve since the beginning of the 12th calendar month
  before that service. This section does not apply to a certificate
  holder that uses only one pilot in the certificate holder's
  operations.

Before that though, 14 CFR 135.323 states:

§135.323 - Training program: General.
...
(b) Whenever a crewmember who is required to take recurrent training
  under this subpart completes the training in the calendar month
  before, or the calendar month after, the month in which that training
  is required, the crewmember is considered to have completed it in the
  calendar month in which it was required.
...

So let's say that I completed initial training in March.  This says that if I complete recurrent training in February or April that they consider the training to have been completed in March.
So what happens if a year passes and recurrent training is due.  I don't make it in February or March, but the company schedules me for recurrent training towards the end of April.  Is it legal for me to fly in April before I go to recurrent training?

At this point, I don't meet 135.343 (because I am no longer within the required 12 calendar months) but I haven't been to training yet, so 135.323 doesn't really apply.
From my interpretation of the regulations I would say no, however every 135 company that I have ever flown for continues using pilots until they go to school, or stop flying at the end of the late grace month if they still haven't gone.  The POI's have never complained about it.  I feel like I must be missing something.
Is it legal for me to fly 135 trips during the late-grace month??  Note that the same the same scenario applies to 61.58 PIC checks as well, which also have a provision to train during the eligibility period (the month before or the month after it is due).

My experience with a 121 carrier mirrors your experience.  We had at one point a severe lack of sim time that caused almost every pilot needing recurrent for a 3 month window to go into the grace month, with most timing out and getting a paid vacation.  Given the scope of those issues I'm sure our POI was well aware of it and I have to conclude that is an accepted practice, though I don't have any regs to point out.  I recall verbiage in our FOM addressing this, but I'll have to check and update.

Yes, the FAA issued a Letter of Interpretation from their Chief Counsel in May 2015 which covers Section 61.58 proficiency checks and specifically addresses this issue.
It says that a pilot may continue to act as PIC during the grace month under Parts 91, 121, and 135.  It says (in part, and emphasis added by me):

Section 61.58(i) states that, if a pilot completes the proficiency
  check in the calendar month before or the calendar month after the
  month it is due, "the pilot is considered to have taken it in the
  month in which it was due for the purpose of computing when the next
  pilot-incommand proficiency check is due."
You have asked whether the language in paragraph (i) creates a "grace
  month" during which a pilot whose currency has lapsed under paragraph
  (a) may continue to act as pilot in command. The FAA has previously
  stated in a legal interpretation that § 61.58 includes a "grace month"
  for purposes of complying with the 12- and 24-month proficiency check
  requirements. We did not explicitly state, however, that a pilot may
  continue to act as PIC during this grace month. Legal Interpretation
  to Sean Conlin (Feb. 24, 2000). We have indicated, however, that
  similar provisions in parts 121 and 135 create a "grace month" during
  which a pilot may to continue to act as PIC during the month after the
  pilot's currency has lapsed? Legal Interpretation to Gregory S. Winton
  (2006), Legal Interpretation to J. Dennis Vanatta (June 25, 1996).
The FAA intended the grace month to limit disruptions in operations
  conducted under parts 121 and 135 by accommodating pilots who are
  unable to complete recurrent training requirements in the month they
  are due. At the same time, the FAA did not want operators to use the
  grace month as a means of regularly extending a 12-month proficiency
  check to a 13-month proficiency check. For that reason, a proficiency
  check under part 121 or part 135 that is completed in the grace month
  is considered to be completed in the month it is due. The FAA believes
  that the same approach is appropriate for§ 61.58 proficiency checks.
  As such, a pilot may continue to act as pilot in command during the
  month after a§ 61.58 proficiency check is due. However, when a pilot
  completes a §61.58 proficiency check during this grace month, it is
  considered to have been completed during the month it was due for the
  purpose of calculating the next proficiency check.

You mention that it includes FAR Part 91.  No where in the text does the part 91 appear.
So lets say the pilot gets their last recurrent Jan 10 of the previous year.  Of course this is just an example.  Their recurrent would be due in January of the following year.  Now this pilot just happens to have two months of vacation available.  They schedule that vacation in January through February of the next year.  Training would put them into class for recurrent in March.
So now their completion date shows March of that year.  When is the period of grace begin (clearly month before and month after) however this training event does on occur anytime within those guidelines.
Is there now a new training month that will be referenced?  Remember this is FAR part 91 and not 135 or 121.
61.58 does not address specifically part 91 events, only the regulated carriers of 135 and 121.
The only sentence that might even be construed to be part 91 would be aircraft that require a PC.

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@AuHunter The Letter of Interpretation that you seem to be referring to does not specifically mention §91, but it does not need to. That letter addresses §61.58, which applies to the PIC of any "aircraft that is type certificated for more than one required pilot flight crewmember or is turbojet-powered" except it "does not apply to persons conducting operations under subpart K of part 91, part 121, 125, 133, 135, or 137
[etc.]". In other words §61.58 applies to operations under §91 except for subpart K. §61.58 does not apply to operations under §121 or §135.

@mins I think this does attempt to answer the question (unless I misunderstand the intent), it just does so inaccurately.

@JonathanWalters: Now I've a doubt about being wrong, I'd like to retract my VTD, but it seems this is not allowed by the review system..


text:
What is the aerodynamic centre and how does it relate to the pitching moment?
Okay so I have been trying to get my head around the Aerodynamic centre and the Pitching moment. Wikipedia really isn't helping much but from what I can gather the Pitching moment is a force caused by the lift from the aerofoil acting upwards from the Aerodynamic Centre (lift) and which acts around the centre of gravity (hence the tail must be configured to counteract this. Can someone please help me out here?

A pitching moment is every moment acting around the lateral axis. Since a moment is the combination of a force and a lever arm, the magnitude of the moment depends on the choice of reference point, the point around which the pitching moment is defined. For flight mechanics, it helps to choose the center of gravity for all pitching moments, because then the weight force will not add a pitching moment of its own. Also, the center of gravity is special because this is the point around which the airplane rotates while in flight.
I guess you mix up aerodynamic center and center of pressure. For me, it helps to use the term "neutral point" instead of aerodynamic center. Why?
Center of pressure describes clearly what it is: If you integrate all the pressures acting perpendicularly to the airflow, the resulting force acts in the center of pressure, just like all masses are summed up in the center of gravity (which, therefore, should be better called the center of mass).
In contrast to the center of mass, this center of pressure is not fixed, but moves when the angle of attack changes. If the airfoil has positive camber, the center of pressure moves forward with increasing angle of attack. If you now move your point of reference away from the center of pressure, you need to replace the lift force by the combination of the lift force and a moment (cogging torque). The motion of the center of pressure is such that at one point the cogging torque is constant regardless of angle of attack and lift. This is the aerodynamic center or neutral point, and I think "neutral point" describes much better what it is: The one point where the cogging torque behaves neutrally regarding angle of attack changes.
The real reason for this name is that static longitudinal stability is neutral (neither positive nor negative) if the center of gravity falls on the neutral point.
In stable flight the pitching moment around the center of gravity has to be zero; otherwise the airplane will display an accelerating pitch motion. To zero the pitch moment, the center of pressure is shifted onto a vertical line through the center of gravity. This shifting can be done by adding or subtracting lift at the tail, which in turn is done by adjusting the elevator deflection or the stabilizer incidence (if the plane has a moveable stabilizer)
Background
In potential flow theory, lift can be calculated as the linear superposition of a contribution from camber and one from angle of attack. While the camber-related part of lift is constant, the angle-of-attack related part varies linearly with this parameter. The center of pressure of the camber part is somewhere at mid-chord (details depend on the camber line; with a Joukowski airfoil the center of pressure is precisely at mid chord). The center of pressure of the angle-of-attack dependent part is at the quarter chord (the center of the area below the chordwise Birnbaum distribution of lift). The important part is the self-similarity of Birnbaum distributions for different angles of attack: The center of pressure of the angle-of-attack dependent part is constant.
Since lift at zero angle of attack is solely from camber, the center of pressure is at around mid chord and moves forward towards the quarter chord point as angle of attack increases. The cogging torque of the camber-related lift contribution around the quarter chord point is also constant, and since the angle-of-attack dependent part of lift has its center of pressure in exactly this quarter point, it will not add a pitching moment of its own at this point. Ergo, the cogging torque of lift in potential flow is constant if the reference point is the quarter chord point.
Fun fact: The center of pressure will move into infinity as the angle of attack is reduced to the zero-lift angle of attack.

I agree, the term "aerodynamic center" is a bit awkward. For me, it helps to use the term "focus" for, I guess, much the same reasons: this is how it's called in my native language :)

Nicely answered.Is there some mathematical way to prove that there must be at least one 'neutral point' where torque is constant through a certain AoA range?

@Radu094: Yes, using potential flow theory. If you add reality, then the answer is no, but potential flow still gives a good approximation of reality as long as flow separation is limited.

@Peter, a tiny nit, although I understand why this is generally presented this way, the reason the CG is the most appropriate choice as a reference point, is because in free flight, any unconstrained body rotates aboiut it's center of mass (a.k.a. Center of Gravity) the force of gravity has nothing to do this this. This principal applies even in outer space, far away from any gravitational force, bodies have a center of Mass and they will rotate about that point if a force is applied to them anywhere other than through that center of mass.

@CharlesBretana: Good point, I added this aspect.

@Peter, in rethinking this, it might be more to the point for me to have phrased it not as I did, but rather, to have said that the center of gravity is misnamed, that this reference point  should have been named center of mass (even in aviation), as it has nothing to do with Gravity, but with inertia, (both linear and rotational), and with the conservation of momentum, (both linear and angular).

@CharlesBretana: That is absolutely right, but language is something inherited, with all the flaws it has accumulated over time. I would prefer to use the more familiar term.

@Peter, I agree... But when it does not seem too pedantic, I like to add the additional details when I speak about issues like this, so as not to explicitly, (or tacitly), exacerbate the common misunderstandings that misnamed or poorly named concepts tend to produce.

@CharlesBretana: I agree again. I edited the text again but hesitate to dwell more on this topic since the question is about the center of pressure.

@Peter, my apologies, I was not urging you to make further changes, just explaining why I make these comments. Gets me in trouble at times. You are a gentleman and a scholar!

@CharlesBretana: The goal here should be to arrive at the best possible answer. I am very grateful for your help in improving this one, so please no apologies.


text:
What is the difference between a pitot tube and a pitot-static tube?
What is the difference between a Pitot tube and a Pitot-static tube? If an aircraft fitted with Pitot tube and static holes on the fuselage, is it necessary to consider the static pressure measurements to calculate the airspeed?

A pitot probe strictly measure ram air pressure, commonly called "pitot pressure" or Pt pressure. A pitot tube measures pitot pressure and pitot pressure only. A separate static port is required to measure static pressure (Ps) and will usually be located on a section of the fuselage where airflow is minimized.
A pitot-static port is a combination pitot AND static port. That is, the tube itself has an opening for detection of Pt pressure and one or more openings for static pressure. In this case, there generally is no static port on the fuselage since it's built into the pitot-static probe.
TLDR: the only difference between the two is that a pitot-static probe is a pitot probe with static ports whereas a pitot tube is strictly just a pitot tube. :-)
Edit: Not sure if the original question was edited after my answer or if I missed the second part in my original response... Static pressure is required to know indicated airspeed because as altitude increases, airspeed increases due to less air resistance.

Also called Total Pressure: both static and dynamic pressure combined.

I haven't heard of that specific item, but Piper used to design the pitot-static systems on their light aircraft with a single blade-like mast which contained both a stagnation tap on the front face and a static port on the aft side as well as a drain hole on the bottom to remove moisture from the unit.  It was also electrically heated for deicing.

Supposedly the Piper design had an advantage over conventional, separate pitot tubes and static ports as installed in Cessnas, Mooneys, etc. in that the indicated airspeed more closely matched that of calibrated airspeed on the pitot-static blades as opposed to the separate tube and port design.

A pitot tube is designed for measuring ram air pressure, but a static port is designed for measuring static pressure.
There are differences between them.
Ram air pressure is the sum of the dynamic pressure and static pressure.
You can't only measure the dynamic pressure because the air always has static pressure.
So you have to measure the ram air pressure which is the sum of the dynamic and static pressure and then remove the static pressure which is measured by static port.


text:
Where can I find data sets on jet fuel prices, including futures and options?
I understand that most airline companies buy jet fuel using hedging instruments such as futures and options. I have been searching on the internet data sets of having both jet fuel and option prices. Can someone give me a link or a research paper which has these data values?

This may be of relevance: https://www.kellogg.northwestern.edu/research/fimrc/papers/jet_fuel.pdf

Airnav.com lists current fuel prices at most airports (that they can get data for). Keep in mind that airlines may (and most likely do) get a bulk discount based on the volumes they buy in.You may want to take a look at this, and keep in mind they may also trade on kerosene futures as that is what jet fuel is made from.


text:
How can I combine flight training hours flown under various legislations in a single logbook?
I am a European citizen and last year I went to the US to start flight training. I got my student license and did my FAA private written exam. I logged about 78 flight hours with a flight school.
I also flew several times with an FAA authorized instructor but I never logged those hours because I was flying part 141 and since my flight instructor was under part 61 I was afraid I could not log those hours. I do not have a private license because I had do go back to Europe.
Now I'm back in Europe but I have the chance to fly at Christmas with an instructor in the US (not a flight school). Could I log these hours on my logbook I had started in the US?
And how can I combine my US and European logbooks?

I'm not really sure what you're asking here. Are you asking if you can add your European flight time to your US logbook? If so, see here and here. And I don't understand your comments about part 61/141 and unlogged time, but perhaps that should be a completely separate question.

Sorry @Pondlife , maybe I'm just not making myself so clear, it's just I'm trying to put a lot of info in such an abbreviated question. If you don't mind I'm going to simplify this: 1) can I still log hours in my American logbook, even tough I'm not flying with a flight school? 2) Can I somehow combine my Euro with my American logbook?

I edited your question, I hope it's still correct but if I made a mistake then you can edit it yourself to fix it.

Based on what you've written, the main issue for you is TSA approval. As a foreigner, you can't receive initial flight training in the US from any school or instructor unless you have a TSA approval in advance. And you need one approval per school or instructor: if the TSA approved you for flight training at a 141 school, you can get training from that specific school only. To use another 141 school or a part 61 instructor, you would need a second, separate approval.
But, assuming that you do have (or can get) the TSA's approval then you can log the training time as usual. The instructor must endorse (sign) your logbook as described in 14 CFR 61.51(h):

(h) Logging training time. (1) A person may log training time when
  that person receives training from an authorized instructor in an
  aircraft, flight simulator, or flight training device.
(2) The training time must be logged in a logbook and must:
(i) Be endorsed in a legible manner by the authorized instructor;

And your question about multiple logbooks has already been asked.

Thank you so much! This was pretty much what I was looking for. Once again, thank you

If I'm reading your answer correctly, without TSA approval he could not legally receive instruction so could only log the time flown if he was solo (which wasn't the case if an instructor was with him).  Correct?

@Lnafziger Correct, AFAIK. If he has a valid solo endorsement, of course.

Your log book is for logging flight time.  You log whenever you fly.  Comments areas help you know where you flew.  Knowing whether hours flown count for a certificate or rating is your responsibility.


text:
Is the Dassault "ministop" related to the Dunlop Maxaret ABS?
Dassault fighters e.g. Ourargan and Mystere had Anti-lock devices called "Ministop" on main landing gear brakes.
Anybody has a picture / drawing / detailed description ?
Is the Ministop related to the Dunlop Maxaret ABS ?

It has been built by Messier. On page 141 and next of this book in French, dedicated to landing gears, the author compares Messier and Dunlop systems: "Ministop consisting of an accelerometer driven in rotation by the wheel and controlling an electrical contact releasing or cutting hydraulic pressure when the deceleration exceeded a defined value (on/off system). Used on Mystère IV, Super Mystère B2, Mirage III, Mirage IV. In 1962 the Ministop was the subject of a testing campaign On Mirage IIIC n°01 at Brétigny CEV"

This 1953 issue of Flight International mentions the Messier Ministop, it seems it was shown publicly for the first time at the Salon de Paris, July 1953:

A particularly interesting development was the new anti-skid device which is being marketed under the name of Ministop. The device consists of an accelerometer—a free mass rotating with the wheel—together with a centrifugal governor and an associated electrical circuit. The general principle of operation is very similar to that of both the Dunlop Maxaret and the Westinghouse Decelostat ...

From that wording I'd say Ministop is unlikely to be a license-built version of Maxaret.


text:
Is there a source of information on the Aerodynamics of Non Rotating Round Disc Wings?
Is there any published information on the aerodynamics of Fixed round disc wings like that used on the Nemeth Parasol?
https://disciplesofflight.com/nemeth-parasol-strange-aircraft/

All I've found so far is info on rotating discs like the Frisbee or Olympic discus.

Yes.
The first published results of measurements made by O. Flachsbart of the AVA Göttingen on a flat disk can be found in the collection Ergebnisse IV (PDF!). He reported on a hysteresis in the angle of attack range between 39° and 42°, where the wing is either stalled or not, depending from which side this angle of attack range is approached.

Lift and pitching moment coefficients of a flat disk, page 97 of Ergebnisse IV
When C. H. Zimmerman tested disk-shaped wings with an Clark Y airfoil section in the early 1930s in the NACA 7 x 10 ft wind tunnel, he called them circular airfoils. The results of this testing can be found in NACA Technical Note 539 (PDF!) from August 1935.

Figures 10 and 11 from the last page of NACA TN 539
Sighard Hörner collected the little data available on circular wings in his book Fluid Dynamic Drag (page 17-8) and concluded:

Disregarding any dreams of "flying saucers", the athletic implement
  called discus seems to be the only practical application of a circular
  planform.

However, Zimmerman went on to inspire the Vought V-173, the "flying pancake" of 1942, from which the Vought XF5U was developed.

Vought V-173 in flight (picture source)

Note: Sighard Horner's comments and information about the discus is on page 17-8 of "Fluid Dynamic LIFT"
https://s3-ap-southeast-1.amazonaws.com/erbuc/files/4099_22ac3a91-02b3-4e33-9e95-4caf350b518d.pdf
Many thanks for the gold mine of information in both "Fluid Dynamic Drag" and "Fluid Dynamic LIFT". These two sources have saved me a huge amount of time searching the internet for answers to specific questions.


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What's is the difference among ECL, CFL, RFL, TFL messages in ATM?
These are several messages to provide a change level during air navigation. Some are sent or requested by the pilot, others by the air traffic controllers, etc. There are subtle differences. Could anybody provide an explanation to show their differences?
Related question: What is the difference between RFL and ECL in Air Traffic Control?

What do you mean by messages? OLDI messages maybe? The answer from GdD in the linked question adequately explains ECL and RFL.

In Air Traffic Control their is quite a few acronyms for levels at various stages in the flight. I am not sure what you meant by messages, or more importantly which messages, so I have kept the answer a little broader.
RFL
Requested Flight Level
The level that is requested in the flight plan before the flight. There can be multiple RFLs in a flight plan. Each time the aircraft wants to cruise at a different level, in the flight plan, an amended RFL is added.
The Pilot may also request an amended RFL mid flight but the RFL is not mentioned at the time just the request for a different level.
Speedbird 1 Request Climb FL360

CFL
Cleared Flight Level
This is the level that the aircraft is currently cleared at or cleared to.

This can be a level on the way to the RFL used as paperstop level for separation (a level the pilot is never expected to maintain but provides separation until conflict no longer exists);
The RFL when the aircraft is maintaining; or
Any other level that the aircraft is cleared to.

ECL
Enroute Cruising Level
Stole part of this one from the question: What is the difference between RFL and ECL in Air Traffic Control?, Thanks GgD for the resource.
From the EUROCONTROL SPECIFICATION FOR TRAJECTORY PREDICTION Edition: 1.0
Page 2:

1.5. Definitions
Enroute Cruising Level - The level that the flight is to maintain for a significant part of the flight after reaching TOC and prior to TOD.

Page 33:

2.3.2  Enroute Cruising Level (ECL) (3.3.2)
Domain Knowledge: The  ECL   is  a  notional
  level  that,  in  the  absence  of  constraints,  equates  to  the
  RFL .  In  the  case  where  a  flight  is  not  permitted  to  cruise
  at  its  RFL   for  any  or  a  part  of  its  route,  due  to  the
  presence of a  strategic constraint (level limit) which applies to
  flights between the  aerodromes of  departure and destination, the ECL
  will be limited  to the level defined by the constraint. The ECL may
  also  be  changed  by  instruction from  ATC where  the flight is to
  maintain a different cruising  level, typically due to the proximity
  of a flight at the  RFL  on the same  route.   As with the  RFL ,
  the  ECL  can be changed for different  segments  of the  route  and,
  if different in  the  cruise  phase,  is  changed  to  the  XFL   on
  passing  the  sector   boundary.  The  notion  of  ECL ceases to exist
  after the final  TOD  from cruising level.

So summarising the above passages, the ECL is the level that will be maintained from top of climb until top of descent. The ECL can be the same as the RFL if the aircraft can be cleared to fly the RFL.
So if there is a conflict at the level of the RFL and the aircraft is cleared to a level below the RFL, the ECL would be that level.
When the aircraft is cleared to fly the ECL the CFL and ECL are the same. So you can get the RFL, ECL and CFL as the same level.
TFL
Personally I have never heard of this level acronym so I am unable to respond on its relationship to the others. If someone else knows I would love to know also.
Additional Level Acronyms
I am also adding the major other level acronyms for ATC into the mix so they are all together. So the answer is as complete as I can get it.
XFL
Exit Flight Level
This is the level that the aircraft is going to cross an ATC sector boundary at. There are two different ways of using the XFL around the world:

In some ANSPs (Air Navigation Service Providers) it is the level that the aircraft will be cleared at when the aircraft calls the next sector. In this case the aircraft could be on Climb or on Descent to the XFL or could be maintaining the XFL all ready. This is used so that the controller knows the CFL when the aircraft calls.
In other ANSPs is is the level that the aircraft will cross the boundary regardless of the CFL issued at the time. This one is used for trajectory calculations inside the receiving ATC system.

PRL
Pilot Reported Level
This is the level the Pilot last reported at. In Procedural Environments this is the only way there is to find out the current level of the Aircraft.
MCL
Mode Charlie Level
There can be other names and acronyms for this level but they all have the same meaning.
This is the level received by the SSR radar or by the ADSB receiver. This has the the aircraft's current level usually embeded in a message with other details. This can be used to fill in the PRL on some systems or is a complete independent system from the PRL.

Sorry I found this question so late but thought it deserved an answer anyway.

Better late than never! I can't vouch for accuracy, but it's certainly thorough.

TFL is the transfer Flight level.
This is the Flight level coordinated between 2 ATC sectors ( I mean 2 controllers ).

This sounds plausible, but could you please add a source? I cannot find the term TFL where I would usually look up such abbreviations. Besides, we normally use XFL and PEL to denote "transfer levels"


text:
Where can I find articles about Handley Page multi element segmented airfoils?
Can anyone direct me to a source containing the complete article on Handley Page's 6 and 8 element airfoils from Flight magazine Feb 24, 1921 (PDF) or further more detailed information on this subject.

Flight magazine Feb 1921 is here.

The images look a lot like the ones in:
A.M.O. Smith, “High-Lift Aerodynamics”, 37th Wright Brothers Lecture, Journal of Aircraft, Vol. 12, No. 6, June 1975
The  classic paper on multi element airfoils, great read!

Ye link breaketh.

Thanks for the heads-up, it should be active again.


text:
Why are there so few aircraft that had inhabited wings?
According to an answer by John Frazer

The only successful plane with inhabited wing might have been the
  Ju-38

Is this true?
In the whole history of aviation there was only one aeroplane with inhabited wings (so that people were able to stay in the wings, not just for repair purposes)?
What are the reasons that there were so few of them?

Modern wings usually aren't that thick, plus it's a good place for fuel and engines to go.

Would the various flying wings, or blended wings, be counted as inhabited?

@jamesqf I wasn't expecting that, but why not

Then the B-2 would count.

Because there isn't much headroom.

This question has some info about why excessive wing thickness has drawbacks.

Off topic, but why do the inboard engines have 4-bladed props, while the outboard ones are 2-bladed?

@jamesqf it's encouraged to ask questions as their own entity. You can link to this question if you think that helps!

@jjack but what is the benefit?

https://aviation.stackexchange.com/questions/46869/why-are-there-different-number-of-propeller-blades-on-the-inboard-and-outboard-e

There are a few more aircraft with seats in the wings, but you are right as long as passenger seats are concerned: they appeared only in the G-38. Now I could also mention the Soviet Kalinin K-7, which was supposed to seat some of its planned 120 passengers in its wing, but that design never went beyond early flight test of the first prototype.
Professor Hugo Junkers was convinced from the start that the best aircraft would only consist of the wing and engines, but the size of real-world aircraft did require a fuselage to place pilot and passengers. Only when wing thickness grows to 2 meters would it be feasible to place people inside (the root thickness of the Junkers G-38 was 1.7 meters, that of the K-7 was 2.33 meters). That was only possible with the largest aircraft of their time, like the Messerschmitt 323 which had engineer's positions in the leading edge of the wing.

The Illustrated London News article about the Me-323 from 1943 contained the picture above (source) which indicates the engineer's cabin between the two inner engines of each wing.
The simple reason for the scarcity of inhabited wings is the limited chord and thickness of practical wings. Back in the days of propeller airplanes, the relative wing thickness could be 20% or more, but with modern jet transports, even 14% are on the high side (this text contains an extensive collection of data). Next, the wing of airliners is full of other stuff:

Leading and trailing edge devices
fuel tanks
pumps, pipes and control linkages

which leave no space for passenger seats. Only in the days of slow aircraft with low wing loadings was the inclusion of passenger seats even conceivable, and only in the largest wings. The G-38 had a wing loading of only 80 kg/m² while the A380 has one of 680 kg/m², so its wing area is only three times larger than that of the G-38 and its maximum root thickness about the same. If you now consider that just 6 out of the 34 passengers on the G-38 could sit in the wings, it should become clear how infeasible it becomes to place passengers in the wing of even the largest aircraft of today.
Junkers went even farther in turning the blended wing into reality when they (together with Messerschmitt) had to submit proposals for a large cargo glider in 1940. While Messerschmitt scaled up their then-largest aircraft, the M-18d, by a factor of three to arrive at the conventional Me-321, Junkers opted for an all-new design which was only loosely based on the G-38 and blended the cargo volume into the center wing. Their Ju-322, however, turned out to be almost uncontrollable. In the haste of the development, nobody cared to check how effective the far too small and too closely coupled tail section was.
Theoretical concepts of aircraft which place their payload in the wings have focused mostly on freighters, like in this NASA study (PDF!).

Just another example...the Boeing 314 Clipper (https://en.wikipedia.org/wiki/Boeing_314_Clipper) did not utilize as extensive in-wing seating, but did have spaces for flight engineers to utilize during flight (https://www.pinterest.com/pin/326862885432596571/).  That said, the low number of passengers it could carry and high costs proved its downfall.

That 1975 NASA paper was an interesting read - and yet there are still no payload in the wings airplane today, nearly 44 years later.  Guess the market place decided fuselage only was sufficient.

@CrossRoads, there is fuel there, which is much easier to put there as it can flow around all the necessary structural elements—and has to be somewhere too.

Ok, I wasn't thinking of fuel as payload, guess I should have. In my plane I figure on 264 or 300 pounds for it (filled to the holes, or filled all the way). The 600 pounds of passengers/baggage is usually of more concern.


text:
What happened to the Ju 52 (“Aunt Ju“)?
I remember when I was a child, my parents and me often saw the Aunt Ju flying. But about 5 years later it disappeared. We never saw it again.
Does anybody know what has been the use of the Ju 52 after the 70s?

During my military service, I have flown in that plane many times... In the early 70s. Today, most surviving units are in museums, but there are a couple of Ju52s still flying sightseeing trips, one of them in Frankfurt, if I remember correctly.

Taste is a personal habit, I love the 747. But I will never understand why so many people love the JU52, it's corrugated surface is really awkward (ugly).

@Peter Probably because it's a trimotor. That's a really unusual setup, there aren't very many models that used it.

Haha. Good, but the definitive answer to three engines is the Lockheed L-1011 (Lucky Ten Eleven) :)

At least for Germany I can assure it still flies!
I live in Germany near Frankfurt and Egelsbach. Frankfurt is probably known, Egelsbach is 20 km south and has a quite large "airport" for private aviation (I don't know the proper term. There are no scheduled flights.)
During summer Aunt Ju can be seen (and heard!) here every few weeks. You can book sightseeing flights (e.g. here or here) or one-way flights. The two Ju52's usually seen here are the "D-AQUI" and the "HB-HOY". They tour throughout Germany until autumn and are then revised until next spring.
An excerpt of the next year's schedule for the HB-HOY shows three sightseeing flights around Egelsbach and Frankfurt, 40 minutes for ~280 EUR (~330 USD) and a one-way flight to Essen (60 mins for 250 EUR).
During the last years the D-AQUI also hopped flew from Egelsbach to Frankfurt (that's just 10 minutes). They say the next year's flight plan for the D-AQUI will be released in January 2018.
I'm sorry that most of the links are in German.

Update (Aug 5 2018):
Sadly, just yesterday (Aug 4 2018) one of the Aunts, HB-HOT, crashed in Switzerland. All twenty people aboard died. They don't know the reason yet.

Update (Jan 27 2019):
Again, sad news: The Lufthansa (which sponsors the "D-AQUI") has decided to stop their sponsorship for the maintenance. They say it's just too expensive for them to keep their Aunt Ju airborne for passenger flights. So "D-AQUI" will not be seen or heard any longer regularly. Unfortunately I have only German reference for this on www.spiegel.de (a major German news magazine).

If you want the "real" Ju, only the Swiss planes will do. Lufthansa changed the D-AQUI beyond recognition: Different engines (BIG difference in sound!), different tires, different control system (don't look into the cockpit; will make you sick!). The list goes on; all in the name of safety and easy part replacement. The Swiss, on the other hand, kept their planes in pristine condition and they run like, well, like Swiss clockworks.

@PeterKämpf Very interesting. I was aware that Lufthansa modified the D-AQUI but not to this extent. Nevertheless I personally prefer seeing the D-AQUI because the Swiss models are often painted with ads which makes them – to me – look like flying suitcases. I must admit though, it's a perfect match.

Back in the Eighties I had a longer conversation with one of the technicians who did the restauration. He did not hide his disgust at the way how Lufthansa handled the work. The pedal unit on the left side is straight from a 737. The pushrods in the original control system had adjustable rod ends, so play and ease of movement could be fine-tuned. He was quite enthusiastic about their quality, but they were all thrown out and replaced by standard aviation-certified parts of much lower quality. And so on. Only the outside is kept in original shape (apart from the number of blades, of course).

Concerning those ads: The person who inherited the Rimowa factory put his fortune into recreating a Junkers F 13. I think he deserves to have his brand prominently displayed on a Ju 52.

According to Wikipedia there are still 8 JU-52's flying today. Which one do you remember seeing flying?
List of airworthy Ju 52s

The JU-52 trimotor is approaching 90 years old.  In its time it was a slightly larger version of the Ford Trimotor.  Corrosion and age are grounding the remaining airframes (Central Europe is no Arizona). The corrugated skin worked very well to improve skin strength.  The cruise speed of around 100 knots did not present excessive drag penalties.  The JU-52 carried a crew of 2 and around 17 passengers.  It's innovative "double wing" slotted trailing edge gave good low speed performance, much like Fowler flaps today.
This type generated some interest when Siemens came out with their electric aircraft motors,  with the possibility of mounting the electric in the nose for takeoff performance and using the gas motors to maintain charge. Maybe they will build some new ones this century.


text:
Sidestick vs. Yoke
Looking at pictures from cockpits of various aircrafts, we can see two types of control-inputs.
The sidestick:

And the yoke:

What are the advantages / disadvantages of both? When is a yoke over a sidestick used? Can the yoke / sidestick do something that isn't possible with the other option?

Initially, aircrafts with side-by-side seating configuration of pilots used the yokes overwhelmingly. The following figure shows the cockpit of Douglas DC-3.

Image from www.wingsovereurope.com
First commercial aircrafts, the WWII bombers and initial jet transports used them. The first Airbus aircraft, A300 had them.

A300 cockpit; image from airlinerworld.com
The yokes have some characteristics like,

They have less sensitivity (compared to sidesticks) and a larger range of motion.
They take up a lot of space and can obscure the instruments in front.
They can be used with either hand.
They can be used pretty much in all types of aircraft- from GA aircraft with conventional controls like Cessna 172 to FBW like the Boeing 787.
One useful thing with yoke is that the pilot and copilot yokes can be mechanically coupled, which is near impossible in a sidestick.
The yoke also provides the other pilot with a visual indication of the control input given.

The sidestick was first introduced in the Airbus 320 along with the FBW. It is doubtful of it would've come into being in aircraft with conventional controls.

The sidesticks differ from the yokes in a number of ways:

The sidesticks are much more sensitive, but have lesser range of travel. They also lend themselves to rapid movements.
Spacewise, they offer significant advantages as they clear up a lot of room in front of the pilot.
In two person cockpits, the sidesticks provide little or no visual indication as to the control input applied; for example, in Airbus aircraft, the sticks move independently and the computer interprets the actions.
They can only be operated with a single hand.
They usually require a FBW system for operation. For small GA aircraft, they are rarely, if ever used.

Very good answer. +1: I didn't think of the problems with sidesticks and FBW.

I heard, that pilots could adapt to use their right/left hand to use the sidestick on either side. From my personal experience I would prefer a neutral input device, a yoke in the center or sidestick in the center. It would be far more natural. I can use the computer mouse on the left side of the keyboard with my left hand...but don't expect me to be the first one in the stats of any first-person-shooter.

"For small GA aircraft, [sidesticks] are rarely, if ever used." I believe they are coming into fashion at least for some light aircraft. The Ikarus C42 has a shared (side)stick between the two seats, for example, and that's all cables, wires and pulleys; likely the only flight control augmentation is the elevator trim. It works quite well.